1. Field of the Invention
The present invention relates to a zoom lens system, and more specifically, to a zoom lens system not requiring any focusing operation and employable for a lens-fitted photographic film package.
2. Description of the Prior Art
Conventionally, an inexpensive camera such as a lens-fitted photographic film package and a lens shutter camera uses a uni focal optical system (e.g. a uni focal lens consisting of one plastic lens) or a bi focal optical system. These optical systems employ a focusing method such as a fixed focus method and a zone focusing method having many advantages. For example, the fixed focus method is a method to enable photographing of a wide range from infinity (.infin.) to a close subject by fixing the subject distance to an excess focal length. This method is particularly superior in operability and cost since focusing in normal photography and a mechanism therefor are unnecessary.
On the other hand, it is desired to use a zoom lens system as the optical system of an inexpensive camera such as the lens-fitted photographic film package. However, since the excess focal length differs depending on the focal length, the above-mentioned fixed focus method cannot be employed for focusing of the zoom lens system. In addition, the normal focusing method as employed for a single-lens reflex camera cannot be employed since it is disadvantage in operability and cost. For this reason, no film package having a zoom lens system has ever been manufactured although the use of the zoom lens system is greatly desired.
Japanese Laid-open Patent Application No. S57-6814 proposes a lens barrel where the excess focal length is applied to focusing of a zoom lens system. In the zoom lens barrel, focusing is performed by a rotation of a distance ring about the optical axis, and during zooming, the distance ring moves straight relative to a fixed barrel along the optical system. Excess focal length graduations which are continuous in correspondence with the focal length during zooming are provided on the periphery of the fixed barrel, and an index which is set at an excess focal length graduation to perform a simple focusing is provided on the distance ring.
However, to set the subject distance at the excess focal length in the zoom lens barrel of Japanese Laid-open Patent Application No. S57-6814, it is necessary for the user to set the index at an excess focal length graduation. Further, this lens barrel is disadvantageous in cost since it is necessarily structured on the premise that a focusing operation is performed. Therefore, this zoom lens barrel cannot be employed for an inexpensive camera such as the lens-fitted photographic film package.
Presently, lens-fitted photographic film packages for various purposes are on the market. For example, in view of purposes such as flash photography, close photography and fixed magnification photography, even though the above-mentioned fixed focus method can be employed for the film having a zoom lens system, the best image cannot always be obtained. The reason therefor will be described below.
According to the fixed focus method, focusing in the normal photographing is unnecessary as described above. However, it is only with respect to a subject at the excess focal length that the best image in the best focus condition is obtained. Nevertheless, in most cases, the subject which the user wishes to photograph the most is not located at the excess focal length, and according to the purpose, the subject is largely away from the position of the excess focal length. Then, the photography possible distance is merely increased haphazardly. In addition, the photography possible distance naturally has its limitation. For example, when a subject the photographer wishes to photograph the most is located closer to the optical system than to the front depth of field, focusing is impossible.
Therefore, when the fixed focus method is employed as a focusing method for a specific purpose, the best image cannot always be obtained according to the focusing of the conventional fixed focus method setting the excess focal length as the subject distance. No solution has ever been proposed to this problem.